Boiler construction



A. M I. MICHELL BOILER CONSTRUCTION June 22, 1954 3 Sheets -Sheet 1 Filed 001.. 28, 1949 m .w ,m

June 22, 1954 A. MOI. MICHELL BOILER CONSTRUCTION 3 Sheets-Sheet 2 Filed Oct. 28, 1949 M 1 p. v .T- FIR/1 MJ a w w N M a w J a a a 7 Jlme 1954 A. M 1. MICHELL 2,681,640

BOILER CONSTRUCTION Filed Oct. 28, 1949 3 Sheet5-$het 3 IN V EN TOR.

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won- 14- Patented June 22, 1954 UNITED STATES .PATENT OFFICE BOILER CONSTRUCTION AlbertMcIlvaine Michell, Warren, Pa.

Application October 28, 1949, Serial Nor 124,149

22 Claims. 1

My invention relates toimprovements in; boiler construction. and more. especially to, vertical updraft fired designof cylindrical or similar shape. The construction 1 describe is designed to meet the special requirement of vaporizing heat sensitive organic liquids, although not specifically restricted to this use.

Vertical fire tube boilers and tubular heaters have been previously described, but the designs usedhave not proven entirely satisfactory in the special. services which involve .heattransf er fluids of organic. composition.

In the conventional water boiler, high combustion, rates and high unitratesof heatabsorption are used. This is possible due to the physical nature of the vaporizing water. Water has a.

high thermal heat conductivity, which assists in absorbin theheat which is delivered to the tube from the flame. or combustion gases. When vaporized, water forms a large volume. of vapor, which promotes rapid. and positive circulation through tube arrangements of considerable complexity. The water. is also notsubject to ready decomposition by. heat, and thermal breakdownis not a problem.

In. orderto provide vapor heating at elevated temperatures (400? F. to 700? F.) and moderate pressures, organic fluids are. often used asrheat transfer media. A typical fluid is Dowtherm, a mixture of diphenyl and. diphenyl oxide. This liquid is often used in boilers, of typical steam type, both fire tube and water tube. Considerable difiiculty may be encountered. in. continuously vaporizing an organic liquid in. aconventional boiler, at temperatures. near the decomposing range. Where the heat from the products of combustion is available at a highrate per unit of surface, the heat isremoved from the heated surfaceby the circulating or vaporizing organic fluid more slowly than would be the case with a steam boiler using Water. The result. is a high film. rise at the heated surface, accompanied by thermal breakdown of the organic fluid and coke formation. The final result is often tube failure due to coking and overheating ofthe metal surface due to nonremoval of the heat to the tube, wall.

The construction which I describe is developed to overcome previous difficulties with thistype of equipment. I combine water tube and fire tube design; In the radiant section, where absorption rates are the highest and temperatures are at the maximum, the heat, is'absorbed. by liquidoirculating inside-of the tubes, with tubes substantially vertical to promote rapid circulation and heat absorption; Inthe convection section, gas flows through the tubes, with liquid infree convection around the tubes, and with lower gas temperatures where. the liquid circulation is less positive. lfhis combination of heating surfaces gives high efflciencies and minimum thermal decomposition.

In addition to the arrangement whereby the combustion gases. flowthrough convection tubes, I show otherarrangements of heat transfer surface to; meet special conditions. The widely varying conditions of-temperatures and fuel costs may dictate different. degrees of efiiciency, and the types of construction which I have shown have specific. application under the conditions as met incommercial practice.

For maximum fuel economy, I describe an arrangement whereby a separatefiuid stream may be circulated through a convection section installed above the boiler drum, thus promoting efficiency.

The construction I describe covers a compact assembly of, tubes, header and drum which may be easily installed in or removed from the enclosing structure, thus facilitating initial construction or repairs in service.

The, relatively small volume of liquid in the boiler, .together with the compact setting, allows quick start up from a cold condition, an important consideration on this type of equipment whenused on process service.

The construction comprising the invention will be understood when read in conjunction with the accompanying drawings, which form part of this specification, and. the reference numerals which are used to designate partsof the various views.

Fig. 1 is a vertical sectional elevation of the boiler construction;

Fig.2 is a. view taken along section 2--2 and looking in the direction of the arrows;

Fig. 3 is aview taken along section 3-3 and looking in the. direction of the arrows;

Fig. 4 is an enlarged detail of connection betweendrum and flue as shown in Fig. 1;

Fig. 5, is a partial. vertical sectional elevation showing a modified construction at the top of boiler;

F g. dis a view taken along line 6-6 looking in direction of the arrows;

Fig. 7 is. a view taken along line 'l--l looking in the direction of. the arrows;

Fig. 8 is a verticalsectional View showing another type of construction-around. boiler drum;

Fi 9 is a vertical sectional elevation taken at the top of the boiler, showing. another type of construction;

Fig. 10 shows a vertical sectional elevation taken around the top of the boiler, showing still another modifiedtype of construction.

Fig. 11 is a view taken along line i l-ll look-- ing in the direction of the arrows.

Referring to the drawings, the boiler consists of a cylindrical shell ll, supported by legs i2, the gases being directed by a fiue or casing 53, to a stack i i. Bolted joints i are provided to allow easy removal of stack and flue.

The shell is lined with suitable refractory or insulation it, to provide thermal protection for the metal shell l l and reduce radiation losses.

I'he heating unit consists of an assembly made up of a bottom header ll, vertical tubes l8, l9, and a top liquid storage and vapor release drum 2%. The tubes H3, is are roller expanded or welded into header ill and drum 28. The bottom header ll rests on base plate H, with cleanout connection 22 installed at convenient points on header, preferably opposite tubes it, it. Where cleanout openings are installed, a hole is cut in base plate for easy access from the outside of the furnace to cleanout door. 7

The burner 23 is shown as a single unit, but multiple burners may be used, depending upon the size of the boiler. The burner or burners are installed in central location on the bottom floor of the furnace, thus giving a uniform heat release with respect to the vertical heating tubes l8, and uniform heat distribution to each tube, since the radiant absorption rate varies with a power of the distance between the source of radiation and the absorbing surface. 7

In the construction shown, relatively large amounts of radiant heat absorbing surface are utilized in order to maintain the unit rate of heat absorption at a relatively low figure, as required for heat sensitive fluids. This results in a relatively low furnace temperature, and under certain conditions, with the burner exposed to the radiant absorption surface, will cool the products of combustion so rapidly as to prevent complete combustion of the fuel.

I therefore propose to install the burner or burners below the floor of the furnace, with liberal combustion space in a burner block or setting installed below the floor line. When the products of combustion pass through the top burner block 2'5, combustion is completed.

Ihe external combustion chamber allows the installation of secondary air control door 24 in a convenient position for regulation.

The close regulation of excess air which is possible with the burner arrangement described, together with the tight furnace setting with this construction, permits the omission of a damper in the stack i l, and still secure low ratios of excess combustion air and high combustion efficiency.

The weight of the boiler assembly made up of header ll, tubes i8, i9, and top drum 29 is supported by resting the header l? on the base plate 25. The lateral movement of this assembly is prevented by bolts 25 which are used to fasten header l? to base plate 25. This arrangement is possible due to the integral nature of the assembly which I describe. The method of support allows free movement of the drum as with respect to header ll. Supporting the weight of the boiler unit at the lower portion of the furnace eliminates the necessity of heavy structural members extending to the top of the furnace, as would be the case if all or part of the weight were supported from above.

The upper drum 213, being supported by circulating tubes l8, I9, is free to expand, and under the high temperatures at which this type of boiler is designed to operate, will have considerable vertical movement, with respect to adjoining cas ing or duct l3.

In the construction I propose, a as seal is desirable at the point of contact of the drum 20 and easing l3. To accomplish this result, I propose a flexible strip 25 which would be attached by metal bars, and bolts 21, or other arrangement to the casing l3 and drum 20, as shown by Fig. 4. This strip 26 would be made up of a thin flexible alloy, or the temperature would allow the use of woven heat resistant cloth, such as asbestos or glass fibre, which could be of the type which is reinforced by metal wire woven into the cloth to increase the strength.

Substantially vertical tubes l8, [9 connect the bottom header ii and the vapor and liquid drum 2.]. The tubes H8, iii are flared outward from the header H to increase the volume of combustion space and remove the tubes from the path of the hot gases. With these tubes in contact with the combustion gases from the burner, additional heat would be absorbed, especially near the bottom of the furnace, which would give high localized absorption rates which are undesirable in the service which I have described.

The arrangement of bent tubes l8, l9 which I show also allows independent expansion of each tube with respect to other tubes, thus preventing excessive expansion stresses from being set up between the return tubes [9 and vaporizing tubes ill, which are non-uniform in temperature.

The arrangement of bottom header ll, burner or burners 23 and flared tubes I8, it, allows the circular header ll to be made in a smaller diameter than would otherwise be possible. It is desirable to keep the size of the bottom header I? as small as possible, thus reducing the amount of liquid at this point. From the standpoint of liquid circulation and short heating period at start up, a minimum amount of liquid storage below the top drum 2B is desirable.

In normal operation, a liquid level is maintained in the top drum 28, as indicated by level gauge 28. A low level alarm 29 is installed on the same connections as the level gauge. When the boiler is at operating temperature, partially vaporized liquid rises by natural convection in heating tubes l8, and is discharged into drum 20 near to or above the liquid level. Unvaporized liquid flows from near the bottom of drum 26 through return tubes l9, to bottom header ll.

Tubes 19 are installed so as to be substantially protected by tubes l8 from direct radiant heating, or where furnace is heavily fired, tubes is may be protected by installing them partially within the refractory wall. In a boiler of this type, with circulating liquid from top drum 2b to bottom header I! at the boiling temperature, any heat absorption in return tubes 19 would cause vaporization and decrease the liquid flow, thus requir- While the rate of heat transfer from thetubes toliquidaround the.

tubes isnot as highas in the. circulating tubes [8, the temperatures are lower and the heat absorption rate from the gases to. the tube. wall is not high, thus preventing. excessive film rise atv the interface of tube wall and liquid. The amount of radiant surface in the tubes I8 is. liberal, in relation to. convection. section, so that ordinarily 60% to 65% of the heat liberated by combustion is absorbed. in the radiant section. Heatabsorption in. the design described, using organic. fluids, is held to a rate of about 2,000. B. t. u. per sq. ft. of surface per hour, or far below typical steam boiler practice.

Vapors from drum.20 are withdrawn through the outlet pipe 31, with external safety valve 32 on line.

The liquid return of condensed vapor in a closed heating system is normally through nozzle 33, unless a special convection section is provided, as hereinafter described.

The gases flowing throughthe convection heating tubes 36 are largely in laminar flow, and. heat transfer rates under these conditions are. not high, perhaps on the order of 2 B. t. u. per hr. per sq. ft. per F. temperature difference. In order to improve heat transfer rates, with resulting improvement in overall eificiency, 1 propose to install spiral fins or swirl strips 34 inside of the tubes. The additional gas turbulence set up by these strips results in greatly improved heat transfer rates; under normal conditions the rate is more than doubled.

A further improvement is to install the fins so that they are welded or otherwise attached to the tube wall. By this means, the heat absorbed by the fins istransmitted directly to the tube wall and thence to the fluid being heated. The effective heat transfer surface is thus increased several fold, and in conjunction with higher rates of heat transfer secured, results greatly improved efficiencies. A considerable percentageofthe available heat is thus. absorbed by a relatively small convection section.

Fig. 5 shows a variation of the construction described. Fig. 6 is a section taken. through line 6--6, in direction of the arrows. Fig. '7 is a section taken through line 'l--'I, in the direction of the arrows.

Under higher rates of operation, the. large volume of combustion gases passing through tubes would set up a high pressure drop. I propose a modified design whereby. the flue casing 13 is installed toleave an annular space. 35 be tween the drum 2B and the casing l3, allowing a portion of flue gasto flow through this annular opening.

In order to proportion the relative flow of gases through tubes 3 0' and. annular space 3.5, I. propose the installation of sliding segmental plates 36, the position of the plates in respect to open annular space 35 being fixed by rods 3'l extending out through openings in flue casing. Two or more plates wouldbe utilized. A draft gauge 38 installed on furnace wall and connecting to the interior, would indicate the pressure within the furnace as compared to atmospheric pressure. The position of plates 36 would be fixed by the draft requirement of the furnace. burner, and available draft from the stack.

With closed heating systems using organic liquids, a considerable initial liquid storage capacity may be required, since on start up the vaporized liquid must be used to fillan external system, thus reducing the liquid level in the drum. The required storage capacity will fix the depth of the drum. The portion of tubes 18 extending upward between the casing l3. and the vertical wall of drum 2! would be substantially outside of radiant heating section, and is therefore not effective as radiant absorbing surface.

With gases bypassed around the drum, as previously described, this: heat transfer surface at the top of the vaporizingtubes I8. would be in direct path of flue gases, and would therefore be available for heat absorption. With. a deeper drum, more tube surface. is installed along the sides. of the drum.

The heat transfer from the combustion gases to the top portion of tubesv l8, when passing through the annular space at this point, could be further improved by the use of extended surface strips 39 on the tubes, which would increase the heat transfer surface and. also the heat transfer rate due to greater gas turbulence- For this purpose, I propose the use of short fiat. strips or round bars welded to the tubes at close. intervals, using for this purpose a spot orfiash welding machine such as is commercially available.

Some heat would also be absorbed by convection between the gases passing along the vertical wall of the vapor drum 2']. The amount of heat absorbed at this point would be considerably increased by welding or otherwise'attaching vertical fins or strips All to. the sides of the drum, thus providing extended heat transfer surface in the direct path of the. gases.

Fig. 8 shows a variationof. previously described arrangements, whereby the entire volume of combustion gases would pass between. the sides of the drum 4| and the duct [3. The drum would be extended upward and formed to general contour of flue l3. Vertical fins 52in form of fiat metal strips or variations thereof may be welded to the sides of the drum, and extending into the path of the fiue gas, give a greatly increased amount of heat transfer surface with corresponding increase in heat absorption.

Another variation of abovedescribed arrangenients would be utilization of" the construction shown in Fig. 1, as modified and shown by Fig. 9, with, a single central flue 43 to carry the combustion gases from the furnace through the drum to the stack. This would give a substantially all radiant furnace, with saving in cost due to simpler construction, although with somewhat lower thermal efiiciency.

The heating arrangement described andutilizing a vaporizing organic liquid, is a closed system, in which the condensed liquid is returned to the boiler by pump or gravity return. Under certain conditions the returning liquid is. well below its boiling temperature, due to subcooling in external equipment. In this case, it would be desirable to utilize the relatively cold liquid to exchange heat with the flue gases at the final point. of exit of the gases, in counter current fiow for maximum efiiciency.

Where temperature conditions justify, I pro pose the installation of an additional convection coil 44 in the flue gas passageabove; the top drum 26. The locationof this coil is shown in Fig. 5, with plan View in Fig; '7. The condensate return or liquid to. theboiler would flow through this coil countercurrent to the combustion gases thus absorbing additional. heat from the gas. The tubes would be installed in one or more hori zontal rows, connected for seriesflow by return bends; or headers. In order to promote heat transfer, the tubes could be. surrounded by circular fins 45 or similar extended surface, tov give greater contact area and higher transfer rates due to greater gas turbulence. One arrangement would be a spiral fin, which would promote turbulence of the gases across the tube banks. With this arrangement using upper convection coil 44, the heated liquid would enter at nozzle l6, flow through the coil, the discharge from the coil to the top drum through a connecting pipe M.

The separate convection coil it could be installed above the vapor drum 20 with multiple tubes 39 for carrying the flue gases through the drum, or could be installed above a single duct or flue 43 through vapor drum, with a casing to confine the flow of gases to the area around the tubes in the coil. (Not shown.)

A further use of the convection coil would be to superheat the vapors leaving the vapor drum 20. In this modification, the vapors would pass from drum 26 through line 41 to coil M, and through nozzle 46 to external use. superheated vapors may be desirable to prevent condensation of vapors in long flow lines.

In many processes using organic vapors for high temperature heating, process steam at superheat temperatures is required, such as in fatty oil deodorization and fatty acid distillation. In some cases, the organic vapor is used to superheat the steam in a separate tubular preheater, or a separately fired superheater is used. I propose an arrangement whereby the coil fi l would be used to superheat steam by passing the steam through the coil, and with steam entering the coil at a lower temperature than that of the liquid or vapor in the boiler, the temperature of the flue gases could be reduced to a lower figure than would otherwise be possible, with resulting higher efficiencies. With this arrangement, steam would enter the coil a l through inlet 48, and out of coil through nozzle 38. Line ll would then serve as the vapor outlet from the boiler.

in the boiler described, the upper portion of the convection heating tubes 39 would be above the liquid level in the drum 28. Greater heat transfer could be secured if the tubes were in contact with liquid throughout their length.

I propose an arrangement whereby the convection or gas tubes 3% would be disposed around the periphery of the drum 2%, in such a manner that each tube would be substantially opposite a circulating tube inle 58. The liquid discharge from tube i8 would flow down gas tube ac, substantially wetting the section in the vapor space and improving heat absorption.

A further modification of the construction I propose is shown in Fig. 10, and plan view Fig. 11 taken through line ii-l I, looking in the direction of arrows.

The drum 2!] would be extended upward to form a dome id. The heating tubes 18 would extend beyond drum 2i] .and connect to dome 49 around the sides. The combustion gases would be diverted by plate 5b through convection heating tubes 36, and would thence pass across the horizontal section of heating tubes it to give additional heat pick up. The heating tubes iii in horizontal section could be bare tubes, or provided with extended surface as shown. The normal liquid level would be carried near the bottom of the dome 49, as indicated by level gauge 28, and the hot vapors are withdrawn from the upper portion of the dome by outlet pipe 3|.

The arrangement described and shown in Figs. 10 and 11 would allow the convection heating tubes 3f! to be completely sealed in the liquid, a

larger vapor release space as afforded by dome 49, and an arrangement of heating tubes l8 to absorb convection heat.

Having thus described my invention, I claim:

1. A vertical circular boiler construction, having a refractory lined shell with a bottom floor, a flue at top, a combustion source centrally located on said bottom floor, an annular header positioned at said bottom floor and shielded from said combustion source, a vapor-liquid drum positioned near the shell top, radiant heating tubes arranged around inside of shell outside the path of combustion gases and connecting said header and drum, liquid return tubes disposed behind the radiant heating tubes within said shell and connecting said drum and header, convection heating means carried by said drum for contact by gas in a passage from furnace to flue, a liquid inlet to said header and a vapor outlet.

2. A vertical circular boiler construction, having a refractory lined shell having a rigid plate bottom, a flue at the shell top, a combustion source centrally located on said bottom, an annular header supported on and by said bottom, a vapor-liquid drum positioned near the shell top, means for sealing said drum to said flue comprising a flexible diaphragm attached to said drum and ilue, convection heating tubes extending through said drum for the passage of combustion gases from said source to said flue, radiant heating tubes arranged around the inside of shell and connecting said header and drum, liquid return tubes disposed behind said radiant tubes and connecting drum and header, said heating and return tubes supporting said drum upon said annular header and independently of said shell, an inlet for liquid into the space defined by said header and said drum, and a vapor outlet from said drum.

3. A vertical circular boiler construction comprising a refractory lined shell having a base plate, a flue at top, a combustion source centrally located on said base plate, an annular header positioned on said base plate, means for attaching the header to base plate to prevent movement laterally thereof, a vapor-liquid drum positioned near the shell top, radiant heating tubes arranged around the inside of shell outside of the path of combustion gases from said combustion source and connecting header and drum, liquid return tubes within said shell disposed behind the radiant heating tubes and connecting drum and header, convection heating tubes extending through said drum for gas passage from said combustion source to said flue, and a liquid inlet to and a vapor outlet from the liquid space 0 said boiler.

4. A vertical circular boiler construction, comprising a refractory lined shell having a base plate, a flue at top, a combustion source centrally supported by and below said base plate, an annular header positioned on said base plate, means for attaching header to base plate to prevent relative movement thereof, cleanout doors in annular header accessible from exterior of said shell through openings cut in base plate, a vaporliquid drum positioned near top, radiant heating tubes arranged'around inside of shell and connecting header and drum, liquid return tubes disposed behind radiant tubes and connecting drum and header, convection heating tubes extending through drum for gas passage from furnace to flue, and liquid inlet and vapor outlet means for said boiler.

5. A vertical circular boiler construction having a refractory lined shell including a'bottom plate, a flue at top, a combustion source at bottom centrally located on said bottom plate, an annular header positioned at bottom, a vaporliquid drum positioned near top, radiant heating tubes arranged around the inside of shell and connecting the header and drum, said tubes bent to extend outward to increase the diameter of the central chamber of said lined shell and to remove tubes from the path of combustion gases or direct flame impingement, liquid return tubes disposed behind the radiant heating tubes and connecting drum and header, convection heating tubes extending through said drum for gas passage from said combustion source to the flue, and liquid inlet and vapor outlet means for said boiler.

6. A vertical circular boiler construction having a refractory lined shell having a bottom plate, a flue at top, a combustion source centrally located on said bottom plate, an annular header positioned at the bottom plate, a vaporliquid drum positioned near the shell top, radiant heating tubes arranged around the inside of the shell and connecting the header and drum, liquid return tubes disposed behind the radiant heating tubes and connecting the drum and header, said tubes being each bent near top and bottom for connection to the drum and header, the bent sections at top and bottom serving to compensate for difierential expansion between radiant heating tubes and liquid return tubes which are at varying temperatures, convection heating tubes extending through drum for gas passage from furnace to flue, and a liquid inlet and vapor outlet for the boiler.

'7. A vertical circular boiler construction having a refractory lined shell including a bottom plate, a flue at top, a combustion source centrally located at said bottom plate, an annular header positioned at the bottom plate, the header closely surrounding said combustion source and being of less diameter than the lining of said shell, a vapor-liquid drum positioned near the shell top, radiant heating tubes arranged around the inside of the shell and connecting header and drum, liquid return tubes in said shell disposed behind said radiant heating tubes and connecting the drum and header, convection heating tubes extending through said drum for gas passage from furnace to flue, and a liquid inlet and vapor outlet.

8. A vertical circular boiler construction having a refractory lined shell with a bottom plate, a flue at top, a combustion source centrally located on the bottom plate, an annular header positioned at the bottom plate, a vapor-liquid drum positioned near the shell top, radiant heating tubes arranged around the inside of shell and connecting the header and drum, liquid return tubes positioned within the shell behind the radiant heating tubes to be substantially shielded from radiant heat absorption, convection heating tubes extending through drum for gas passage from furnace to flue, and a liquid inlet and vapor outlet; said radiant heating tubes and liquid return tubes constituting means supporting said vapor-liquid drum on said bottom plate and independently of the top portion of said shell.

9. A vertical circular boiler construction having a refractory lined shell with a bottom plate, a flue at top, a combustion source centrally located at said bottom plate, an annular header positioned on the bottom plate, a vapor-liquid drum positioned near the shell top, radiant heating tubes arranged around the inside of shell and connecting the header and drum, liquid return tubes positioned within said shell behind radi- 1 ant heating tubes and shielded by said refractory lining, convection heating tubes extending through the drum for gas passage from the combustion source to the flue, and a liquid inlet and vapor outlet; said radiant heating tubes and liquid return tubes constituting means supporting said vapor-liquid drum on said bottom plate and independently of the top portion of said shell.

10. A vertical circular boiler construction comprising a refractory lined shell having a bottom plate and terminating at its top in a combustion. gas flue, a short flame fluid-fuel combustion source centrally supported by and below said bottom plate, an annular header centrally supported on said bottom plate, means shielding said annular header from the flame zone of said combustion source, a vapor-liquid drum positioned near the shell top, a circular array of vertically extending radiant heating tubes arranged around the inside of shell outside of the path of combustion gases and connecting the header and drum, the tubes being connected to the drum above normal liquid level, liquid return tubes disposed within said lined shell behind the radiant heating tubes and connecting the drum and header, convection heating tubes extending through said drum for gas passage from said combustion source to said flue, a liquid inlet into said header, and a vapor outlet for said drum.

11. A vertical circular boiler construction as recited in claim 10, in combination with a tubular coil positioned above said drum in the path of combustion gases, and means for connecting said coil to said drum.

12. A vertical circular boiler construction as recited in claim 11, wherein said tubular coil is provided with extended heat-absorbing surfaces.

13. A vertical circular boiler construction as recited in claim 11, wherein said tubular coil is provided with continuous spiral fins.

14. A vertical circular boiler construction as recited in claim 10', in combination with a tubular coil positioned above said drum in the path of combustion gases, said coil having inlet and outlet openings at the exterior of said shell.

15. A vertical circular boiler construction as recited in claim 10, wherein said vapor-liquid drum comprises a main portion of larger diameter through which said convection heating tubes extend and a smaller diameter vapor dome extending above said main portion and within the space defined by said convection heating tubes, and said radiant heating tubes extend around said main portion of the vapor-liquid drum and are connected to said dome.

16. A vertical circular boiler construction as recited in claim 15, wherein the portions of said radiant heating tubes which are positioned above said main portion of the vapor-liquid drum are provided with heat-absorbing fins.

17. A vertical circular boiler construction as recited in claim 10, wherein swirl strips are provided in said convection heating tubes.

18. A vertical circular boiler construction as recited in claim 10, wherein said drum is spaced from said flue by an annular passage through which there is a flow of combustion gases, in combination with heat absorbing means on said radiant heating tubes where contacted by combustion gases.

19. A vertical circular boiler construction as recited in claim 18, in combination with segmental plates movable to regulate the effective cross-section of said annular passage.

20. A vertical circular boiler construction as recited in claim 1, wherein said convection heating means comprises heat-absorbing fins on said drum.

21. A vertical circular boiler construction as recited in claim 1, wherein said convection heating means comprises heat-absorbing fins on said drum; the drum being spaced from the flue by an annular passage for flow of combustion gases.

22. A vertical circular boiler construction as recited in claim 1, wherein said convection heating means comprises a single axial duct through the drum for flow of all combustion gases therethrough.

References Cited in the file of this patent UNITED STATES PATENTS Number 5 93,380 215,670 226,880 414,297 664,534 821,329 987,863 1,068,301 1,171,900 2,334,968 2,529,611

Name Date Whitely Aug. 3, 1869 Purinton May 20, 1879 Quinn Apr. 27, 1880 Culver Nov. 5, 1889 Connelly Dec. 25, 1900 Beckley May 22, 1906 Norris Dec. 29, 1908 Bettington July 22, 1913 Still Feb. 15, 1916 Throckmorton et al. Nov. 23, 1943 Kallam Nov. 14, 1950 

